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TVS breakdown voltages for higher currents

Discussion in 'Electronic Design' started by Joerg, Sep 22, 2012.

  1. Joerg

    Joerg Guest

    Hello Folks,

    Does anyone know a manufacturer that provides a wee bit more info in
    datasheets for their TVS?

    The issue:

    We have SMAJ78CA in a board because long cables, filters and such can
    cause nasty ringing. The board must tolerate several 100msec bursts of
    80V amplitude in a row, but only on rare occasions and then such a burst
    is a one-time event. Long story short, these things won't clamp well
    before 100V is reached in the ringing and then some stuff with 100V abs
    max ratings can go phhhhut.

    Unfortunately min-max breakdown voltage for such TVS is only spec'd at
    1mA which isn't helpful. If there was a spec for, say 20mA, that would
    be different. I have inquired but not much hope. Does anyone know a mfg
    that specs also at higher currents? Must be in the tables and not just
    typical graphs because this product goes into a safety-critical and thus
    regulated market.

    Tried a few SPICE models such as the ".SUBCKT 1smb70at3g" from:

    http://www.onsemi.com/pub/Collateral/1SMA70AT3G.LIB

    But they all seem a bit on the primitive side, either a simple diode
    model or piecewise linear, and also no min-max.
     
  2. Littlefuse.


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  3. Joerg

    Joerg Guest

  4. Guest

    Epcos has V/I characteristics of their TVS products over decades of current:
    http://www.epcos.com/inf/75/db/CTVS_09/Leaded__SHCV.pdf
     
  5. Fred Bartoli

    Fred Bartoli Guest

    Joerg a écrit :
    I doubt you'll find anything like that. Plus their tempco will give you
    even more hard time.

    Your mention of 20mA seems to indicate that your working impedance isn't
    too low, which would help...
    I had that sort of problem once and I resorted to 2 inductor-TVS stages
    (had to withstand 8/20 surge levels) followed by a resistor/zener clamp.
    My impedances where high (supplying CMOS logic)

    In you case maybe you can do that, or a TVS clamp followed by a zener
    driven voltage limiter...
     
  6. Joerg

    Joerg Guest

    I was able to get that for a zener once. But it was a smaller
    manufacturer and they are often more flexible. They actually gave me
    their QC acceptance criteria sheets and they tested at many more current
    values than the one listed in the datasheet.


    Actually it is unknown and can be very low. What I mean is that if the
    TVS is guaranteed to run at no more than, say, 20mA at 80V we'd be fine.
    Even 50mA would be ok. This is because those spikes will be short and
    seldom.

    Probably that is the case for a TVS with 70V standoff voltage or
    possibly even for lower ones. But I need hard data for this design.

    I sure would have liked to provide an electronic clamp using a big old
    FET but the problem is we did not have the required board space.
     
  7. Joerg

    Joerg Guest

    I was able to get that for a zener once. But it was a smaller
    manufacturer and they are often more flexible. They actually gave me
    their QC acceptance criteria sheets and they tested at many more current
    values than the one listed in the datasheet.


    Actually it is unknown and can be very low. What I mean is that if the
    TVS is guaranteed to run at no more than, say, 20mA at 80V we'd be fine.
    Even 50mA would be ok. This is because those spikes will be short and
    seldom.

    Probably that is the case for a TVS with 70V standoff voltage or
    possibly even for lower ones. But I need hard data for this design.

    I sure would have liked to provide an electronic clamp using a big old
    FET but the problem is we did not have the required board space.
     
  8. Joerg

    Joerg Guest

  9. Joerg

    Joerg Guest


    He who works on mission-critical electronics shall be cautious about
    that. You can't step in front of an NTSB inspector and say "But the
    graph showed that this should normally not have happened!".


    TINSTAAFL ;-)

    Not "is", it's "ain't", man :)
     
  10. Guest

    Many of these flea-sized TVS have really weak average power handling capability, usually just 10s mW, so those 100msec burst times of yours have a good chance of blowing the SMAJ78CA. You better start with the repetitive ratepower handling of the part before you waste a bunch of time looking for V/I limit guarantees.
     
  11. Joerg

    Joerg Guest

    SMA isn't exactly flea-sized. They can take a watt or two for a few
    hundred msec. That's all I am asking them to do.

    [...]
     
  12. Joerg

    Joerg Guest

    Zen wasn't possible, I had about the square-footage of a fly to clamp at
    two places, no chance to have opening devices because they'd have to be
    quite major.

    Anyhow, there will never be much energy above 80V and all I want is
    something that is guaranteed not to conduct for than a few tens of
    milliamps at 80V. The normal voltage range is less than 32V and the 80V
    peaks will be very short and a rare occurrence. I am fairly sure a 70V
    TVS can do that but a bench test isn't good enough here.
     
  13. Guest

    No they can't. The Vishay ds says repetitive power handling of 300W at 0.01% duty which comes to 30mW.
     
  14. Joerg

    Joerg Guest


    Of course not continuously. That's why I wrote "for a few hundred msec".
    As mentioned before, the duty cycle will be extremely low, and it'll be
    less than 0.01%.

    I am not going to sink 300W pulse into them but maybe 1-2W pulses.
     
  15. Guest

    Okay , from Fig 5. http://www.mouser.com/catalog/specsheets/smaj50.pdf the transient thermal impedance is less than 10oC/W (~7) for the 100ms-1000ms pulses, so it's a non-issue. But anything approaching continuous and Rja goes towards 120oC/W in that flea package.
     
  16. Robert Baer

    Robert Baer Guest

    As i vaguely remember, a TVS is a crappy limiter, with a slow
    limiting slope and high incremental resistance reminiscent of that seen
    for a 2.7V or 3.3V zener or like that of a forward biased diode.
    Use a real zener with a current limiting resistor in series so that
    it will not blow out.
     
  17. Joerg

    Joerg Guest


    That is exactly what I meant. It is a non-issue _if_ you know how much
    it will conduct at 80V, worst case maximum. And I don't, hence my
    inquiry and my post here.
     
  18. Joerg

    Joerg Guest

    Zeners would be (somewhat) better but, for example, a 82V zener in an
    SMA package can only take 55mA of surge current and only for less than
    10msec non-repetitive:

    http://www.vishay.com/doc?85782
     
  19. Nemo

    Nemo Guest

    We have SMAJ78CA in a board because long cables, filters and such can
    You could try a gas discharge tube as a first line of defence, there are
    some tiny surface mount ones around now which trip around 70V, and then
    clamp around 20V. Can take amps for much longer than a TVS.
     
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